1. Field of the Invention
The present invention relates to a permeability detection apparatus that detects permeability detection such as toner concentration or the like on a non-contact state or to an electric conductivity detection apparatus that detects electric conductivity of an electric conductor, which is provided in a PPC (Plain Paper Copier) and an LBP (Laser Beam Printer), for example.
2. Description of the Prior Art
Generally, in an electrophotographic copier, an electrifier evenly electrifies a surface of a photosensitive drum, a static latent image is formed by performing exposure to a photosensitive body based on image information, toner is selectively adhered on the static latent image to perform development, a toner image obtained is subjected to fixation after it is transferred on a plain paper, and thus a final image is obtained.
FIG. 2 shows a configuration example of a conventional development apparatus. The development apparatus is a two-step development roller method of a small diameter. In this method, an upper development sleeve and a lower development sleeve are disposed adjacent to the photosensitive drum, and a paddle roller supplies developer composed of non-magnetic toner, which mainly consists of coloring agent, and magnetic carrier onto the surface of photosensitive drum. Since the toner reduces although the magnetic carrier reduces little as development is executed, a toner hopper is provided to replenish the toner reduced, in which toner for replenishment is housed.
Density of a developed image becomes light when the mixture ratio of the toner to the magnetic carrier reduces, and it becomes opposite when the mixture ratio increases. To obtain an image of appropriate quality, the toner housed in the development apparatus always needs to be maintained within a constant level range, and a toner concentration detection apparatus (T sensor) for detecting toner concentration in the developer is installed for this purpose. Further, an agitation roller is provided in the development apparatus to make a mixture state of the developer and the toner for replenishment uniform.
There exists technology disclosed in Japanese Patent Laid-Open (unexamined) No. 6 (1995)-289717 as a conventional example of a concrete circuit configuration to detect the toner concentration in a development apparatus, that is, permeability. In a magnetic detection apparatus for detecting the toner concentration in this gazette, a detection coil adjacent to the developer and a reference coil remote from the developer are connected in series and driven by an alternating current drive source, a phase difference between a differential output voltage at a connection point of both coils and a voltage of the alternating current dive source is calculated, and thus detecting magnetic fluctuation in the developer. Specifically, it is the one that uses a phase difference detection of a differential transducer type.
Further, there also exists technology disclosed in Japanese Patent Laid-Open (unexamined) No. 2000-131120 as a prior art to detect with good sensitivity and stably whether or not a toner amount in the toner hopper has reduced to a reference amount. According to this technology, the development apparatus is arranged near a coil L1 and a coil L2, which are magnetically coupled with respect to each other, capacitors C1, C2 respectively connected to the coils L1, L2 in parallel are provided, the coil L1 and the capacitor C1 form a first resonant circuit, the coil L2 and the capacitor C2 form a second resonant circuit, and the two circuits constitute a tuning circuit as a whole. Then, the tuning circuit is connected to a solid-state oscillation circuit at its input side via an amplification circuit, a reference voltage that is an output of the tuning circuit and its level are compared in a comparison circuit after they are input to a wave detection circuit, and whether or not a toner level has reached the reference level is thus detected.
In this known example, when the toner level of development apparatus is approximately equal to the reference amount, a resonant frequency of the resonant circuit approximates to an oscillation frequency of the solid-state oscillator by magnetic influence of the magnetic carrier at the toner level, and thus impedance is the minimum as the tuning circuit. Furthermore, since the capacitor C1 is connected to the coil L1 in parallel to form the resonant circuit, and a Q value of the tuning circuit is set high together with the resonant circuit of the coil L1 and the capacitor C2, the toner level can be detected with high sensitivity. In the technology disclosed on the gazette, it is described that the toner amount is detected quantitatively and an analog output can be made.
In Japanese Patent Laid-Open (unexamined) No. 6 (1995)-289717 of prior art, it discloses phase difference detection by the differential transducer using the detection coil and the reference coil, which is characterized in that a resistance for sensitivity adjustment is connected to either one of the coils in parallel and toner concentration is stably controlled by gradually changing the phase of differential output voltage relative to sudden inductance change of the detection coil side.
In the technology of the gazette, a core of magnetic material in the differential transducer needs to be adjusted individually corresponding to a magnetic material detected because the both coils and the alternating current power source forms the oscillation circuit. Moreover, since it is difficult to oscillate stably, the oscillation frequency is as low as a few hundred Hz, and the coil L or the capacitor C needs to be large, there exists a problem in manufacturing in a small size and a low manufacturing cost.
Furthermore, Japanese Patent Laid-Open (unexamined) No. 2000-131120 of prior art describes that the toner concentration can be detected by using the solid-state oscillation circuit, the first resonant circuit and the second resonant circuit, and toner concentration detection is extracted as an output level of the tuning circuit that consists of the first and second resonant circuits.
As described, since changes of output amplitude in the tuning circuit are extracted as an output signal by existence/non-existence of the magnetic material (a phase difference signal is not detected), both accuracy and sensitivity are insufficient. Thus, there is a problem that analog detection of a continuous amount such as the toner concentration is difficult although existence/non-existence of the magnetic material might be possible. Moreover, although it is also disclosed that oscillation of the oscillation circuit is stabilized by allowing the amplification circuit to position between the oscillation circuit and the tuning circuit, the amplification circuit having a large number of parts needs to be specially prepared.
An object of the present invention is to provide a permeability detection apparatus in which a detection operation of permeability such as toner concentration is stably executed and the sensitivity of permeability detection is improved.
To achieve the foregoing object, a permeability detection apparatus according to one embodiment of the present invention includes: a detection circuit having a circuit element, which includes a coil arranged in a region near the magnetic material where permeability shall be detected, a capacitor connected to the coil in series or in parallel, and a resistance connected to the coil and the capacitor; an oscillation circuit that oscillates a natural frequency to drive the detection circuit; and an output circuit that extracts a signal from the detection circuit to output a permeability signal detected. The resistance has a function to allow the oscillation circuit to eliminate influence from the detection circuit and to oscillate the natural frequency continuously.
Further, a permeability detection apparatus according to another embodiment includes: the detection circuit having the circuit element, which includes the coil arranged near the magnetic material, the capacitor connected to the coil in series or in parallel, and the resistance connected to the coil and the capacitor; the oscillation circuit that oscillates the natural frequency to drive the detection circuit; and a phase comparison circuit that extracts signals from an input side of the detection circuit and an end side of the circuit element to compare the phases thereof. The resistance has the function to allow the oscillation circuit to eliminate influence from the detection circuit and to oscillate the natural frequency continuously.
Further, in the permeability detection apparatus of another embodiment, the coil and the capacitor form the resonant circuit and a resonant frequency of the resonant circuit is set approximately equal to the natural frequency.
Furthermore, a permeability detection apparatus of still another embodiment includes: a detection circuit that has a coil, which consists of a first coil of the first resonant circuit and a second coil of the second resonant circuit magnetically coupled to the first coil, first and second capacitors respectively connected to the first and second coils in series or in parallel, and a resistance connected to the first resonator circuit; the oscillation circuit that oscillates the natural frequency to drive the detection circuit; and the phase comparison circuit that extracts signals from an input side of the detection circuit and an output side of the second resonant circuit to compare the phases thereof. Resonant frequencies of the first and second resonant circuits are set approximately equal with each other and the resonant frequencies are set approximately equal to the natural frequency, and the resistance has the function to allow the oscillation circuit to eliminate influence from the detection circuit and to oscillate the natural frequency continuously.